The fate of CO2 at extreme conditions

IME researchers recently achieved a milestone in the understanding of carbon deep within the earth. Using first principles simulations, Ding Pan and Giulia Galli have revised the current models of the dominant species of carbon dioxide (CO2) in water under extreme conditions. This constitutes a major development in understanding the so-called deep carbon cycle, which has critical application to global climate models as well as fundamental molecular physics. The results appeared in Science Advances on October 12th, 2016.

For decades, standard geochemical models have assumed that aqueous CO2(aq) is the major carbon species present in water at pressure/temperature conditions of the earth mantle. Pan and Galli have instead found that most of the dissolved carbon at these conditions exists in the form of solvated carbonate (CO22- and HCO3) anions. In fact, under such extreme conditions, even H2CO3(aq) is more abundant than CO2(aq), a dramatic difference with respect to ambient conditions.

“Water is a major component of geofluids, which help transport great amounts of carbon in the Earth’s interior. However, we still do not fully know the forms of dissolved carbon in water at such extreme conditions. Our study shows that carbon-bearing fluids cannot be simply modeled as the mixtures of neutral gas molecules. Rather, they may exhibit complicated ionic interactions in water at these high pressure and high temperature conditions,” said Ding Pan, the first author of the Science Advances article.

While it is well known that the autoionization of water is greatly enhanced at extreme conditions, e.g., at the P-T conditions of the Earth’s mantle, and that the pH of neutral water may be well below 7, it has long been unclear how the notable changes of water properties under pressure affect the solvation of oxidized carbon. Using ab initio simulations, this study demonstrates that the coordination number of carbon changes in water at the Earth’s upper mantle conditions. It also finds that ion pairing between Na+ and CO32- and HCO3- is greatly affected by P-T conditions, decreasing with pressure at 800 -1000 K. These results suggest that at extreme conditions water transports carbon mostly through highly active ions, not CO2(aq) molecules. An interesting corollary to these results is that CO2 degassing likely happens close to the Earth's surface and not deep within the Earth.

Understanding molecular behavior at extreme conditions has long been a goal of experimental physics and materials science. “Experiments at these conditions remain extremely difficult, both in execution and interpretation. Fortunately, developments in theory and increases in computing power have recently brought ab initio simulation to the forefront of the field,” said Galli, the senior investigator of the paper. Leading the effort, Pan & Galli’s study on the fate of carbon dioxide in water at extreme conditions is a critical contribution to both experimental and computational research on the molecular behavior of deep carbon.

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